1. The Field of the Invention
This invention relates to a reverse osmosis or ultrafiltration membrane cartridge having an improved central collection tube.
2. The Prior Art
Reverse osmosis and ultrafiltration membrane cartridges have often been produced by winding a membrane leaf or leaves, a permeate transfer sheet, and a spacer sheet around a perforated central collection tube. The permeate transfer sheet collects permeate passing through the membrane leaf or leaves for passage through perforations or apertures in the collection tube into its interior, and thence through the collection tube out of the pressure resistant casing in which this may be housed, to facilities for use, disposal or storage. Such cartridges are known as spiral wound cartridges and are described in U.S. Pat. Nos. 3,417,870; 3,493,496; and 3,542,203, among others.
The collection tube in a spiral wound cartridge should be fabricated of relatively rigid material, preferably a strong plastic such as polyvinyl chloride, since it is the central hub or core around which the other cartridge elements are wound. Additionally, it must be provided with a series of perforations or apertures for transfer of permeate from the permeate transfer leaf into the interior of the collection tube. These perforations must be of such dimensions that adequate permeate flow-through is provided while a generally smooth outer tube surface is maintained for membrane pack support. Preferably, the perforations are in the form of small holes or transverse slits. These requirements have heretofore been met by a central collection tube fabricated from a continuous length of commercially produced plastic tubing in which the required number of transfer passages are provided by drilling holes or milling slits through the tube wall. While the raw material, that is the plastic tubing, is not very expensive, the labor required for drilling or milling the number of transfer holes or slits needed is great, and the total cost of the finished central collection tube is high. A central collection tube construction which could be more readily and economically fabricated would be advantageous for production of spiral wound membrane cartridges for reverse osmosis or ultrafiltration systems.